1. Technical Field of the Invention
The present invention relates to optical network elements and, more particularly, to an acousto-optical fiber Bragg grating filter (FBGF) having a wide tuning range and a method of making the same.
2. Description of Related Art
As networks face increasing bandwidth demand and diminishing fiber availability in the existing fiber plant, network providers are migrating towards a new network technology called the optical network. Optical networks are high-capacity telecommunications networks comprised of optical and opto-electronic technologies and components, and provide wavelength-based services in addition to signal routing, grooming, and restoration at the wavelength level. These networks, based on the emergence of the so-called optical layer operating entirely in the optical domain in transport networks, can not only support extraordinary capacity (up to terabits per second (Tbps)), but also provide reduced costs for bandwidth-intensive applications such as the Internet, interactive video-on-demand and multimedia, and advanced digital services.
Of the several key enabling technologies necessary for the successful deployment of optical networks, two are particularly significant: dense wavelength division multiplexing (DWDM) and Erbium-Doped Fiber Amplifiers (EDFAs). DWDM is a fiber-optic transmission technique that has emerged as a crucial component for facilitating the transmission of diverse payloads regardless of their bit-rate and format over the optical layer. DWDM increases the capacity of embedded fiber by first assigning incoming optical signals to specific wavelengths within a designated frequency band (i.e., channels separated by sub-nanometer spacing) and then multiplexing the resulting signals out onto a single fiber. Because incoming signals are not terminated in the optical layer, the interface is bit-rate and format independent, allowing service/network providers to integrate the DWDM technology with existing equipment in the network.
By combining multiple optical signals using DWDM, they can be amplified as a group and transported over a single fiber to increase capacity in a cost-effective manner. Each signal carried can be at a different rate (e.g., Optical Carrier (OC)-3, OC-12, OC-48, etc.) and in a different format (e.g., Synchronous Optical Network (SONET) and its companion Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Internet Protocol (IP) data, etc.).
Current advances in DWDM technologies allow a large number of wavelengths to be multiplexed over a fiber using sub-nanometer spacing. For example, up to 32 channels or carriers may be spaced 100 GHz apart (equal to 0.8 nm) in a multiplexed optical signal operating at around 1550 nm. In contrast, some of the standardized, xe2x80x9ccoarsexe2x80x9d wavelength separations include 200 GHz spacing (1.6 nm) and 400 GHz spacing (3.2 nm), both at around 1550 nm.
Several advances are also taking place in the field of optical amplifiers which operate in a specific band of frequency spectrum and boost lightwave signals to extend their reach without converting them back to electrical form. To optically amplify the individual wavelengths of multiplexed signals, optical amplifiers need to have a gain bandpass that extends over the entire range of the DWDM signal""s bandwidth. For example, for 32 channels with a spacing of 0.8 nm around the 1550 nm band, the signal bandwidth is about 26 nm and, accordingly, the spectral gain profile of the optical amplifier should cover at least this range. Advanced optical amplifiers such as the EDFAsxe2x80x94which have a gain profile of about 30 to 50 nmxe2x80x94are currently being employed in optical networks using DWDM transmission techniques.
Those skilled in the art should readily recognize that in order to fully realize the benefits of such advances as DWDM techniques and EDFAs in optical networks, the ability to separate the individual wavelengths in a multiplexed optical signal is critical because these wavelengths typically need to be routed to individual detectors at the end of the transmission. Although various optical filtering technologies are currently available for this purpose, there exist several drawbacks and deficiencies in the state-of-the-art solutions.
For example, wavelength separators using interference filters and Fabry-Perot filters typically have a low resolution which renders them a poor choice for the sub-nanometer spacing of the current DWDM techniques. Further, these filters do not have a quick enough response time for achieving any degree of tunability, that is, the ability to select different wavelengths using the same filter, in a practical manner.
Optical filters made of fiber Bragg gratings offer excellent resolution characteristics. However, current fiber Bragg gratings are typically provided as xe2x80x9cinherentxe2x80x9d gratings wherein the grating is xe2x80x9cwrittenxe2x80x9d into optical fibers as a fixed structure such that tuning is possible only by altering the length of the fiber on a macro scale. In general, such fixed Bragg gratings allow tuning over a few nanometers only, which approximates to about 5 or 6 channels. Clearly, this tuning range is insufficient to cover the channel bandwidth of the advanced DWDM systems described hereinabove.
Based upon the foregoing, it should be apparent that there is an acute need for an optical filter solution that provides a wide tuning range for selecting wavelengths among a large number of channels available in today""s DWDM systems. Additionally, it would be advantageous to have a narrow optical passband (for the selected wavelength) so as to be able to tune to a particular wavelength more precisely without optical crosstalk effects. It would be of further advantage to provide the capability for tuning over a range that is at least co-extensive with the gain profiles of the advanced EDFAs used in current optical networks. The present invention provides such a solution.
Accordingly, the present invention is discloses a wide range tunable acousto-optical filter and a method of making the same. An acoustic transducer is provided for generating an acoustic pressure wave of a selected frequency that is propagated longitudinally along an optical fiber member. The pressure wave generates a plurality of alternating localized compressions and rarefactions in the refractive index (RI) of the optical fiber, thereby creating an RI profile. The periodic changes in the RI profile operate as a grating with a corresponding pitch for reflecting optical signals of a particular wavelength (i.e., Bragg resonance wavelength). The acoustic pressure wave""s frequency is modulated by controlling the acoustic transducer such that a variable grating pitch is correspondingly obtained, thereby causing a change in the Bragg resonance wavelength of the grating. In response, a reflected optical signal selected from incoming multiplexed optical signals tunes to a different wavelength. A closed-loop controller is provided for controlling input signals to the acoustic transducer/actuator so as to modulate the tuning of the reflected optical signals.
In one exemplary embodiment, the acoustic actuator comprises a discrete transducer that is coupled to an optical fiber. In other exemplary embodiments, the acoustic transducer comprises a section of the optical fiber having a piezoelectric effect or electrostrictive effect. The acoustic pressure wave""s frequency is preferably controlled by varying the frequency of the electrical signal that is supplied as input to the transducer.
In another aspect, the present invention is directed to a method of filtering an optical signal in a fiber. An acoustic transducer is driven at a selected frequency to propagate an acoustic pressure wave longitudinally in the fiber. The acoustic pressure wave generates a plurality of alternating localized compressions and rarefactions in the fiber so as to effectuate a grating therein. The grating operates to reflect optical signals of a particular wavelength based on the periodicity of the plurality of alternating localized compressions and rarefactions. Accordingly, the wavelength of the reflected optical signals is controlled by modulating the selected frequency of the acoustic pressure wave.
In yet another aspect, the present invention is directed to a tunable optical filter which includes a fiber having a selected length and RI for conducting optical signals. An acoustic transducer is associated with the fiber for propagating an acoustic pressure wave of a selected frequency through the fiber such that the acoustic pressure wave alters the local RI by generating a plurality of alternating localized compressions and rarefactions in the fiber. A grating is accordingly effectuated in a selected portion of the fiber, wherein the grating reflects optical signals of a particular wavelength based on the periodicity of the alternating localized compressions and rarefactions. A closed-loop controller is provided in association with the acoustic transducer for modulating the frequency of the acoustic pressure wave so as to vary the grating period.
In yet further aspect, the present invention is directed to a method of making a tunable optical filter having an integrated acoustic transducer. A silica tube (preferably pure amorphous quartz) having a selected length and a selected diameter is provided. A core material is deposited by chemical vapor deposition (CVD) inside the silica tube for forming an optical fiber. Thereafter, a selected portion of the optical fiber is heat-treated while under a predetermined electrical stress so as to form a piezoelectric transducer in the selected portion. Upon actuation, the piezoelectric transducer propagates an acoustic pressure wave of a selected frequency along the optical fiber to generate a plurality of alternating localized compressions and rarefactions in the optical fiber so as to effectuate a grating therein. The grating""s pitch corresponds to the period of the localized compressions and rarefactions, and is modulated to reflect optical signals of a particular wavelength by controlling the frequency of the acoustic pressure wave.